1. Field of the Invention
This invention relates to a method and apparatus for monitoring the flue gases of an industrial combustion process. A method and an apparatus for sampling and continuously monitoring flue gases from an industrial furnace is disclosed. The method is particularly adapted to processes with hostile environments, characterized by high particulate levels and high temperatures and/or vibrations, such as electric arc furnaces (EAF). The method is also adaptable to any industrial combustion processes where flue gas monitoring is desired. Exhaust gases of a furnace are sampled through an extractive probe and analyzed through a remote optical device, preferably a diode laser, for measurement of flue gas species concentration. Thus, an extractive sampling system is integrated with a non-intrusive optical measurement technique that is resistant to the problems encountered by prior art flue gas monitoring systems. This facilitates more optimal furnace efficiency, as post-combustion of the CO and/or H2 byproducts of the production process can be achieved.
2. Description of the Prior Art
In many industrial furnace operations, and in particular in the recycling of scrap steel in electric arc furnaces, large volumes of carbon monoxide (CO), hydrogen (H2) and/or other combustible gases are frequently vented as components of the flue gases from the furnace. These gases contain heating value that can be recovered to improve furnace efficiency and reduce the cost of steel recovery. These combustible gases form a significant proportion of the waste gas leaving the furnace, which also normally contain a large amount of particulate dust and liquid or semi-liquid slag droplets.
Because of the large amounts of energy used in an electric arc furnace, steel mill operators are increasingly looking for ways to use all potential energy during the steel making process. Hence, a variety of schemes exist to further react the CO and H2 with oxygen prior to their exit from the furnace, to form carbon dioxide (CO2) and water vapor (H2O). These post-combustion reactions of CO, H2 and oxygen are highly exothermic, and therefore release heat to the solid scrap, the slag and the melt phases in the furnace, thus allowing to recover part of the heat content of the gases and to save electrical energy.
To burn the combustible gases, oxygen is injected into the furnace. In order to determine the initiation of, duration of and rate of such oxygen flow, it is necessary to continuously analyze the waste gas (by measuring its oxygen or carbon monoxide content, for example) and to use the resulting measurement to open or modulate oxygen admission valves. Because these post-combustion systems operate on a feedback principle and that combustion in EAF is a very dynamic phenomenon, they must respond quickly to any change in the waste gas composition.
Two major methods for gas composition monitoring in harsh, high particle laden industrial combustion processes can be found: 1) extractive sampling techniques coupled with gas analyzing systems and 2) in situ optical measurement techniques.
Conventional extractive sampling methods for gas composition analysis is performed using a temperature resistant water-cooled probe, a water removal system, primary and secondary particle filters, and a suction pump to withdraw the sample from the process. These components are assembled before the analyzers to ensure the sample is cooled and free of water and particulate matter, which could otherwise damage the analyzer equipment. Since many industrial environments are not suitable for sensitive instrumentation, sampling lines from the process to the analyzer can be up to hundreds of feet in length, introducing a delay that can be up to tens of seconds before reaching the analyzer. Then, analyzers exhibit a characteristic response time that will add to the measurement delay.
Additionally, the waste gas at the position from which samples are normally taken entrains a high concentration of dust and other particulate matter. The particulate matter impacts on the probe and tends to agglomerate thereon, as well as in gas lines used to draw the gas sample to the analyzer. This occurs often to the extent that the probe becomes blocked, necessitating replacement and/or cleaning of the probe.
Thus, while conventional sampling apparatus may be effective in obtaining a single gas sample every few hours, heretofore sampling apparatus have proven ineffective in the continuous collection of samples of furnace flue gases, as they become clogged after extended periods of use. Moreover, many of the existing extractive probes have a limited lifetime due to exposure to high temperatures and high particle density streams, as in the case of EAF installation where they are located in the break flange of the flue gas system.
An alternative to this method, which has been developed to overcome the delay time and maintenance issues associated with extractive sampling techniques, is dealing with in situ optical measurements, viz., absorption techniques. This method and the related apparatus have already been documented in U.S. Pat. No. 5,984,998. Since optical measurements are non-intrusive, no gas extraction is required and thus measurements can theoretically be conducted in the harshest environments containing high particle density atmospheres. This reduces the maintenance issues due to probe plugging and corrosion, as well as the associated cost.
Practically, optical systems based on absorption use sources and detectors located at opposite sides of furnaces. A beam of radiation is launched across the process, thus creating an optical line-of-sight through the flue gas exhaust duct.
Moreover, a unique feature of the method is the fast-time response that allows real-time monitoring of the combustion atmosphere. However, experience from field trials conducted using this optical approach allowed identifying several technical issues that must be taken into consideration. First, in addition to molecular absorption along the optical path, the laser beam may be significantly attenuated in its passage through the flue gas due to scattering caused by the very large number of small particles entrained in the gas flow.
In some cases, the attenuation of incident radiation can be so great to extinguish it completely, thus receiving no exiting radiation and voiding the measurement. Measurement quality can also degrade due to mechanical vibration of the beam launch and receiver. Industrial processes with high vibration will steer the beam on and off the detector resulting in signal noise.
In cases where transmission is acceptable but high particle densities are present, baseline quality of the measurement degrades due to temporal effect of the radiation particle interaction. Another form of noise also due to the beam moving on and off the detector is beam steering resulting from the refractive index gradients in high temperature process streams. It is thus clear that, depending on the process environment under consideration, such in situ optical techniques can significantly loose their accuracy, to the extent that it becomes impossible to obtain a reliable measurement.
Hence, no reliable, accurate and real-time method is currently available for monitoring flue gas species concentrations in high-temperature, high-particulate combustion processes (or at least in electric arc furnaces), which has prevented so far the dynamic control of such processes and the optimization of their efficiency.
Thus, a problem associated with methods for sampling and continuously monitoring flue gases from an industrial furnace that precede the present invention is that they provide a slow response time and thereby do not adequately indicate process conditions to enable optimal process control.
A problem associated with in situ optical measuring techniques for flue gases from an industrial furnace that precede the present invention is that they do not provide point measurements, and therefore do not allow sampling the flue gas specifically in the desired locations of the exhaust stream.
Still another problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they do not facilitate optimal use of the potential energy present in the process by-products for heat generation.
Yet another problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they are susceptible to probe plugging and corrosion.
An even further problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they require undue replacement of the monitoring equipment.
Still a further problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they do not provide continuous, near real-time measurements of the species concentration in the waste gases, with acceptable accuracy, so as to facilitate an adapted dynamic monitoring of process characteristics.
Another problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they do not adequately facilitate control of the CO and H2 combustion by-products.
An even further problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they do not facilitate monitoring of most or all species whose concentration can be measured by absorption techniques, e.g., H2, CO, O2, H2O, CO2, NOx and/or SOx.
Yet another problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they cannot be used in industrial processes that experience high particle densities, temperature gradients, mechanical vibration, rapid variations in temperature and gas composition, and high radiation loads from the process itself.
Still another problem associated with methods for monitoring the flue gases of an industrial combustion process that precede the present invention is that they have a complex electro-mechanical structure, are expensive to construct and difficult to maintain.
In contrast to the foregoing, the present invention provides a method and apparatus for monitoring the flue gases of an industrial combustion process that seeks to overcome the foregoing problems and provide a more simplistic, more easily constructed and relatively reliable methodology.